This achievement is tricky. While it can be obtained in any zone, the first zone is probably best-suited for attempting this achievement as there are less enemies on-screen and they come less frequently than in other zones. Use the flak cannon for small enemies (1 shot=1 kill=100% accuracy). It is inevitable that you will fail to land a few shots, so switch to your machine gun for larger enemies and bosses (landing every bullet makes up for the flak-cannon misfires). Don't bother with the Lairs when going for this achievement, and if you are forced to go on-foot refrain from firing until you are back in your vehicle. Also refrain from shooting bunkers or other structures, as the bullets you fire on them are considered wasted. This achievement can be unlocked on any difficulty, and will unlock when you finish your current zone, NOT when you meet the requirement.